Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 28
Текст из файла (страница 28)
It is for thisreason that most production CV D reactors today operate as batch systems.One alternative is to operate a single-wafer system, but do it in a continuous150Production CVD Reactor Systems151fashion; two such systems will be described. Finally, if the process is sufficientlysensitive, it may be acceptable to operate a single-wafer system without thecontinuous operation, and several newer systems will be reviewed later.It is important to recognize that a production reactor is not simply a reaction chamber.
If it is a low pressure unit, there will be a vacuum system whichcan be quite complex. There will be a gas panel which regulates gas mass flowinto the chamber. The method of heating the wafers and/or the entire chamberhas to be chosen carefully. Wafer transport involves many tradeoffs, and forbatch systems if any degree of automation is required, will be quite involved.Finally, most production reactors these days operate under microprocessorcontrol, and quite a lot of software must be developed.In the balance of this chapter, we will review a number of reactors thatare currently in production use. As much as possible, the above features ofsuch reactors will be discussed.6.2 LOW-TEMPERATURE SILICON DIOXIDE REACTORSOne of the earl iest production-ready CVD reactors was developed to depositlow-temperature Si0 2 at atmospheric pressure. 1 Rather than a batch system,Appl ied Materials, Inco designed a system with a large area of uniform reactantflow and moved wafers continuously through this zone.
The wafer remainedin this reactant flow long enough to achieve the required film thickness, typically 1 micron. A block diagram of this reactor is shown in Figure 1, showingthe major subsystems.EXHAUSTLAMINAR FLOWAUTO LOADtttt_1-LAMINAR FLOWAUTO STACKit~ ~ l ~ ~~~~~~AAN~D- -.....GAS SUPPL Y _ _......0.Figure 1: Continuous atmospheric pressure CVD reactor. 1152Chemical Vapor Deposition for MicroelectronicsThey are:(1) Reactant/purge gas disperser(2) Heater/temperature control(3) Wafer transport(4) Wafer tray load/unload(5) Reactant/purge gas flow control(6) Reactor exhaustWafers are transported, in Inconel trays, on a moving belt through a reactantgas flow while being heated from below by quartz radiant heaters.
Reactantgases are introduced through a unique disperser head design. Details of thegas introduction are shown in Figure 2.SIDE VIEWPREHEATERMAIN HEATERTOP VIEWN 2 CURTAINDEPOSITION AREAEDGE ZONEEDGE ZONEPRE·HEATMAIN ZONEDEPOSITION MAIN ZONEEDGE ZONEEDGE ZONEFigureN 2 CURTAIN N 2 PURGE2: Gas flows in AMS-2000. 1As the wafer tray (which can hold up to three wafers) enters the reactor,it passes through a nitrogen purge and curtain, so that the wafers heat up inan inert atmosphere before being exposed to the reactant gases. It also prevents air from entering the reaction zone.
After deposition, the wafer trayexits through a similar nitrogen and purge. The flow and temperature profiles along the wafer path are shown in Figure 3.Production CVD Reactor SystemsSTABLE'S~b~~~GillPU~~EIDEP~~~.'g~ ~:SESGASES DISPERSED: :FROM ABOVE:PtfjU~~E:COOLDOWNCURTAIN~0I[SfABLE ISOLATIONEPUR!2:PREHEATCURTAIN153GAS FLOWSLM/INCHFINALPURGE\X _c:IJ1/ PRE-PURGEDEPOSITION ZONEWAFER _WAFERTRAYS ~~~c==::::;:;a~C;;:;;=;;;;;;;;~~~===::I~~TRAYSLOADED ~MAIN HEATER ZONE~ UNLOADEDdII, _Z =====:JPREHEATZONEPOSTHEATPREHEATZONECOOLPOSTHEAT DOWNZONEZONE 500400DEPOSITION ZONE",,--,r------------r-.--.o..,JWAFEATRAYHEATED FROMBELOWi~~~300200 IOCl100INCHES 03691218212427303336 39 4245 48I~-"""""---------II-~IPHASE IPHASE IIPHASE IIIFigure 3: Gas flow diagram-Appl ied Materials.The reactant gas disperser is assembled from anodized aluminum platesof alternating geometry.
A cross sectional view normal to the flow directionillustrating the gas flow pattern is shown in Figure 4.DISPERSION HEADD - - - - - - -0HEATERFigure 4: AMS-2100 gas flow pattern.!154Chemical Vapor Deposition for MicroelectronicsThe gases are introduced via five perforated tubes positioned in the flowdirection, so that a uniform flow can be established. A short flow path tothe wafer surface is chosen to minimize any gas phase reactions.
The reactantgases are rapidly removed from the reaction zone and exhausted downward.A photograph illustrating the deposition zone of an AMS-2100 is shownin Figure 5. Here, the N2 purge regions and the exit from the dispersion headcan be clearly seen.Figure 5: Deposition region of AMS-2100. Applied Materials.Approximately once each shift, the disperser head has to be cleaned. Avacuum cleaner is used to remove Si0 2 particles that have built up on thedisperser lower surface.This basic system was designed to deposit Si0 2 from the Si H4 + O 2 reaction at about 400°C and atmospheric pressure.
It can also deposit dopedoxides by introducing PH 3 for phosphorus doping or B2 H6 for boron doping.In order to protect personnel from these toxic dopants, the reactor is housedin a vented enclosure.Due to the high deposition rates possible at atmospheric pressure, approximately 1000 A/min, wafer throughput can be as high as 200 to 400 per hour.Also, since this is an atmospheric pressure reactor, there is no expensive vacuumsystem, and the capital cost of the reactor system is modest. These two factscontribute to a low cost per wafer processed, and has allowed this system toremain in commercial use for over 13 years.The shortcomings of such a system are that some of the Si0 2 particlesformed on the disperser head wind up on the wafers.
As IC dimensions haveshrunk over the years, the ability to tolerate even very small particles has lessened, and other approaches have been developed.A second atmospheric pressure system has seen some considerable commercial success in recent years. It is based on the concept of mixing the reactants at the substrate surface, following the concept illustrated in Figure25 of Chapter 1. 2 As before, the wafer travels through the region of reactantgas flow on a belt, passing through nitrogen curtains.
A sketch of the CVDchamber of this reactor is shown in Figure 6. As we see, the reactive gases enterthrough three feed lines, and they are not mixed until they exit the ejector.As soon as these gases impinge on the wafer, they are exhausted to the vent.Prod uction CV D Reactor Systems155INCOMINGGASES+Itt,!TO"VENTTOVENTIIIIIINJECTO RI,....-.---.-1II!I~I !1\~ ~\II ,\.PURGE~:=::========~fLAPPE1I DOORSIII::C::===::;;::=::::;;;;;:::'=:;';;;;=====:=~:=:::::;;:S;:::U:::;fh;;;;;ST~T~ ~E3l(a)REACTANT G.~SESMETERING· SLOTSiH 4N2SEPARATOR°2,(SUBSTRATE)(b)Figure 6: Atmospheric pressure CVD system with gas injector. 2Pressure in the purge plenums is arranged to be slightly higher than pressureat the exit to the injector. In this way, none of the reactive gases can flowanywhere but the vent.By minimizing the residence time of the reactive or spent gases above thewafer, the designers of this reactor attempt to minimize gas phase reactionsand subsequent generation of particles.
Since the SiH 4 and O2 are introducedunmixed just above the wafer at high velocity, deposition rates as high as 10,000156Chemical Vapor Deposition for MicroelectronicsA/min are achieved. This permits a throughput of 96-4" wafers per hour witha 6" belt.Since the chemistry is very similar to the previous system, SiO z , PSGor BPSG fil ms can be deposited at temperatures of approximately 350° to450°C. It is very interesting that films deposited by this technique exhibitedlower tensile stresses than other techniques, about 1 x 10-9 dynes/cm z .The primary advantage of this reactor over the Applied Materials Systemis that it requires less power, and the dispersion head is easily removed forcleaning.
However, the AMS-2100 has about half the footprint, so it takes upless valuable clean room space. As far as film quality is concerned, both systems produce comparable films and particles remain a concern in each system.6.3 HOT TUBE, LOW PRESSURE, THERMAL SYSTEMSIn Chapter 2, we reviewed the concept of carrying out CVD processesat low pressure so that deposition becomes surface controlled. When the onlything controlling the uniformity of deposition is the temperature of the wafersurface, all we have to do is ensure that the wafer is in a uniform temperature furnace. Again, at low pressures, the diffusion coefficient is so large thatwe can stack wafers up next to each other so 50 to 100 can be placed in a longtubular furnace.Because of this arrangement, it has become economically desirable todeposit.
poly, SiO z and Si 3 N4 in such furnaces, and there are many commercial versions of this type of furnace. A typical LPCVD furnace is shown inFigure 7.-.a , _-_.a..I-.::~Figure 7: Tylan-Tytan automated LPCVD reactor.Production CVD Reactor Systems157This system contains a four-tube stack of furnace tubes for even higherproductivity. Wafers are stacked in a quartz boat which can be loaded fromleft to right into the furnace tube. In any CVD system, deposition occurs onthe inside of the tube, and it is not economical to clean the tube after eachdeposition.
The film that forms on the tube can be scraped as the boat is inserted, and this leads to particles which end up on the wafers. To avoid this,the wafer boat is loaded by a cantilever arrangement such as one shown inFigure 8. In this figure, we also see how each furnace tube is constructed.An electrical resistance heater with more turns at the tube ends (to compensate for heat losses) surrounds each tube.
There is a vertical laminar flowhood over the loading area to minimize particle contamination of the wafersbeing loaded. As we can see, there are temperature controls for the furnacetubes, and a power module to provide the electrical power. When operatedas a LPCVD system, a unit including both the gas flow and vacuum systemsis positioned on the right side.
Such a unit is shown in Figure 8. Here we cansee the vacuum pumps on the left, and the mass flow controllers on the right.The vacuum pump oil recirculation systems are shown in the slide out drawers.As can be seen in Figure 9, this system, as well as most current similar systems, operate under computer control.As before, the major design challenge in such LPCVD systems is to maintain uniform film quality and deposition rate on all wafers in the system. Sincethe reactant gas flow can deplete as it travels down the tube, it is not alwayseasy to achieve this goal with tight tolerances. In some cases, changing thetemperature along the tube can compensate for an axial change in depositionrate.(..,.,,,tf'· ......
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
